Ultrasonic intrusion detection systems typically transmit ultrasonic energy into a region to be protected and detect intruder presence induced Doppler-shifted ultrasonic energy received therefrom to provide an alarm signal indication of unauthorized intruder presence. Transmission and reception is typically accomplished by ultrasonic transceivers that have electro-mechanical components commonly including vibrating membranes, piezoelectic crystals, and housing mounting members. The physical integrity and therewith the performance of such components tends to deteriorate with age, and often in such a way that produces failure and/or false alarm situations if allowed to develop undetected and unchecked.
Typical electrical components for the transceivers include a crystal oscillator and intruder presence detection circuitry that are usually electrically interconnected to the transceivers by elongated wires. Vibration, solder contact deterioration, and other factors often so disturb the electrical wires from their intended interconnection points as to produce undesireable open-circuit conditions in the transceiver feed and receive paths as well as to produce undesireable electrical short circuit paths in the transceivers and associated electronic detection circuitry.
Another source of false and failure of alarm situations for ultrasonic intrusion detection systems is undetected and uncompensated changes from nominal in the atmospheric conditions of the sound propagation medium. Excessive pollution, extreme temperature changes, and atmospheric pressure changes, among others, may so alter the acoustic propagation medium that the actual system range either over-extends or under-extends the nominal range thereby occassioning false alarm situations and failure of alarm situations.
A further impediment to the utility of ultrasonic motion detection systems is presented by the ability of objects located in the nearfield of the transceivers to prevent energy transmission and reception in such a way as to effectively circumvent intruder motion detection. Such an event could occur, for example, by an intruder who gains access to the location of the ultrasonic transceivers and places an object in the radiative path thereof as by cupping it over by hand.